Metal radionuclides are frequently used in nuclear imaging and therapy. Fluorescently labelled ligands are commonly used for optical imaging in animal models or in vivo imaging during i.e. surgical interventions. Related tracers are usually formed from a metal binding group (chelate ligand) or fluorescent units and moieties, which are bound, with or without linkers, to one or more targeting vectors.

Currently, DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) is the most frequently used compound for the purpose of nuclear imaging with radiometals. It forms stable complexes with many transition metal ions as well as lanthanide ions. Very frequently, one of the acetic acid side arms of this molecule is used to for conjugation of DOTA to the targeting vector resulting in the formation of an amide. For imaging purposes, the metal ion is finally added in the last step, thereby forming the complex which serves as the tracer or radiopharmaceutical. Particularly for complexation of lanthanide ions, the most established chelators are DOTA and derivatives of this structure, such as 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A). For smaller metal ions such as Ga(III), bifunctional chelators based on the 1,4,7-triaza-1,4,7-triacetic acid (NOTA) structural motif, such as NODAGA (1,4,7-triaza-1-glutaric acid-4,7-diacetic acid), have recently gained popularity because the resulting complexes are more thermodynamically stable and are formed more easily.
All of these chelators bind the radioactive metal ion by coordination on the nitrogen atoms of the azamacrocycle backbone and on the deprotonated carboxyl groups of the acetic acid substituents. These carboxylic acid moieties thus have to be deprotonated in order to act as coordination sites, which is why a pH value exceeding their pKa of approx. 3.5-4.5 must be maintained during the labelling procedure. Labelling at a lower pH is substantially hampered. In case of the radionuclide 68Ga(III) this is somewhat contradictory due to the fact that at pH>3, formation of colloidal Ga(OH)3 commences which is also inhibiting the complex formation. For labelling of all iminopolyacetic acid ligands, like the above mentioned ones, a careful adjustment of pH value to 3-3.5 is thus mandatory. In addition, labelling of DOTA-like structures requires either heating, usually up to 80-95° C., or comparably high ligand concentrations, in the range of for example 1 mM.
In order to prepare bioconjugates of chelators, that is, molecules consisting of a targeting vector covalently bound to a chelating unit, the use of protecting groups on either side is mandatory in most cases according to the prior art. Particularly in case of DOTA or DO3A, the carboxylate moieties intended for metal complexation are often protected during amide coupling, see for example Schottelius M, Schwaiger M, Wester H J. Rapid and high-yield solution phase synthesis of DOTA-Tyr3-octreotide and DOTA-Tyr3-octreotate using unprotected DOTA. Tetrahedron Lett. 2003, 44 (11), 2393-2396. Tris-tert-butyl esters of these compounds are thus employed for conjugation. In most cases, an additional subsequent deprotection step is necessary in order to obtain the desired fully deprotected bioconjugate.
In the context of bioconjugates, the term multimer refers to molecules which comprise more than one targeting vector of the same kind. Multimers are desirable because in comparison to monomers, they can exhibit increased affinity to the respective target, thus often resulting in higher target uptake, higher target/background ratios, and thus better images. For preparation of multimeric radiometal tracers, DOTA-like chelators are usually bound to a linker which allows more than one targeting vector to be bound. The assembly of such molecules usually involves multistep syntheses with low overall yield. In addition and especially in the case of Ga-68 labelled peptides, two carboxylic groups of DOTA have been used for the formation of dimers by amide formation.
Combination of more than one imaging modality is usually referred to as multimodal imaging. Imaging technologies that can be combined include positron emission tomography (PET), single photon emission computed tomography (SPECT), planar scintigraphy, fluorescence imaging, magnet resonance tomography (MRT), optical imaging (either fluorescence or bioluminescence imaging) and combinations thereof, i.e. PET/CT and PET/MRT. To perform multimodal imaging, tracers are required possessing more than one reporter unit, e.g. combining the presence of a chelate unit for radiometals (for e.g. PET) with a so called fluorophor (for fluorescence). For such an approach, the presence of targeting vectors in the same molecule is, however not mandatory, still necessary for functional imaging.
Recently, a novel chelating unit based on 1,4,7-triazacyclononane, PrP9 (I), has been introduced, see for example Notni J., Hermann P., Havlickova J., Kotek J., Kubicek V., Plutnar J., Loktionova N., Riss P. J., Rosch F., Lukes I., A Triazacyclononane-Based Bifunctional Phosphinate Ligand for the Preparation of Multimeric Ga-68 Tracers for Positron Emission Tomography. Chemistry—A European Journal. 2010; 16(24):7174-7185.

3PrP9 (I) is particularly suitable for complexation of smaller radiometal ions, such as 68Ga3+. In contrast to the above mentioned chelators bearing carboxylates, phosphinic acid moieties exhibit pKa values below 1. Thus, labelling can be performed at pH<2, thus circumventing formation of colloids.